Nonaqueous electrolyte secondary batteries (hereinafter also referred to as “nonaqueous secondary battery”) such as a lithium secondary battery are currently in wide use as batteries for devices such as a personal computer, a mobile telephone, and a portable information terminal.
A device equipped with a lithium ion battery includes multiple electrical protection circuits provided in a battery charger or a battery pack, in order to ensure a normal and safe operation of the battery. For example, if a breakdown or a malfunction, occurs in the protection circuits, the lithium ion battery can be continuously recharged. This can cause oxidative and reductive degradation of an electrolyte on the surfaces of a cathode and an anode both of which generate heat, oxygen release caused by decomposition of a cathode active material, and even deposition of metallic lithium on the anode. This can eventually cause the lithium ion battery to fall into a thermal runaway. There is also a danger that ignition or explosion may occur in the lithium ion battery, depending on the situation.
In order to safely stop a battery before such a dangerous thermal runaway occurs, most lithium ion batteries currently include a porous base material, which serves as a separator, containing polyolefin as a main component, the porous base material having a shutdown function of clogging pores present in the porous base material when a temperature inside the battery is raised due to some defect and reaches approximately 130° to 140° C. Exhibition of the shutdown function at a temperature rise inside the battery stops passage of ions in the separator and thus allows the battery to safely stop.
On the other hand, the porous base material containing polyolefin as a main component is poorly adhered to electrodes. This may cause decrease in battery capacity and decrease in cycle characteristics. With the aim of improving adhesiveness of the porous base material to electrodes, there has been developed a separator in which a porous layer containing a polyvinylidene fluoride-based resin is laminated on at least one surface of the porous base material.
For example, Patent Literature 1 discloses setting a porosity of a porous layer containing a polyvinylidene fluoride-based resin to 30% to 60% and setting an average pore diameter to 1 nm to 100 nm with consideration for adhesion to electrodes and ion permeability.